Welding techniques in which a blanket of inert gas is maintained over the weld area have been available for many years. These use torches that have either a fixed tungsten electrode or a consumable metal electrode with a blanketing gas that is normally argon or helium. In comparison with the usual arc welding procedures, the inert gas welding torches are normally used in closer proximity to the weld zone. Plasma jet torches using an exceedingly hot stream of ionized gas to effect fusion are used in a similar manner. These torches are subject to a very high heat environment which tends to rapidly degrade the outer layer of insulation used over the electrically conducting interior metallic elements. Part of this degradation is due to physical damage from the cuts and scrapes which inevitably occur during normal use. This is greatly exacerbated when the insulation is softened from the heat and it becomes much more subject to abrasion and cuts. The net result of insulation degradation is a prematurely shortened life to an otherwise useful torch.
The usual insulating materials for inert gas or plasma welding torches are heat resistant phenolic molding materials or an elastomer such as a rubber composition. Both are usually applied by compression molding or otherwise forming the polymeric material around the preassembled metallic elements of the torch. Silicone rubber materials in particular, which are generally preferred because of their excellent heat resistance, have extremely poor flow characteristics. By this is meant that they require very high pressures and temperatures to make them conform to a mold without leaving knit lines and void areas. They are usually supplied as uncured sheets which are then cut into strips and hand wrapped around the metallic torch components prior to placing this assembly in a compression mold. Usually a considerable amount of excess rubber is required to make parts of acceptable appearance. This excess is squeezed from the mold as flash which is then trimmed after the part has been removed and cooled somewhat. After the molding cycle, the part is then placed in an oven for a relatively long period to complete cure of the rubber. The curing process induces cross linking of the heat resistant polymers so that they become thermoset; i.e., they cannot be molded further by heat or pressure.
Even in their fully cured state the rubber jackets are relatively soft and lack the toughness of vulcanized carbon reinforced synthetic rubbers such as those used in tires and hoses. As noted above, the insulation on a welding torch is subject to damage from abrasion or cuts simply due to the rough environment in which it is normally used. This is in addition to deterioration due to the heat. Further, as heat deterioration begins to take place mechanical damage occurs more readily and ultimately the rubber coating will begin to spall off then break out in chunky pieces.
Attempts have been made previously to deal with the problem of insulation reinforcement. Muehlberger, in U.S. Pat. No. 3,823,302 shows a plasma jet torch that, without providing further elaboration, uses an insulating member described as being "a fiber glass-resin composition". Harvey, in U.S. Pat. No. 3,610,866, describes a torch with a conductive metal element encased in an electrical insulating sleeve "such as fiberglass coated with silicon rubber". Again the inventor provided no further information other than the brief phrase quoted.
Attempts to reinforce silicone rubbers or other rubber-like elastomers such as neoprene with fibrous glass mats or woven fabric have not been very successful in the past. This is due to the poor flow characteristics of the rubbers caused by an exceedingly high viscosity under molding conditions. Appearance has been very poor for this reason. It is highly unlikely that a successful product could be made, as Harvey seems to suggest, by first wrapping the torch with glass reinforcement and then applying the rubber as a wrap or in some other manner. Metal pans to be coated with elastomers must first be primed to ensure good adhesion of the rubber to the portion being covered. If the glass was placed under the rubber; i.e., adjacent to the metallic parts as Harvey appears to teach, flow of the rubber through the glass mat would likely be so poor that there would be minimal, if any, bonding between rubber and metal.
The present invention is directed to a process of insulating a torch with a heat curable polymeric jacket that overcomes the problems noted and produces a product of excellent appearance and durability.